Fluorescent lamps, and devices for driving them, are commonly known, therefore a detailed explanation is omitted here. FIG. 1 is a schematic block diagram, showing a fluorescent lamp L and a driver 1 for driving this lamp. The driver has an input terminal 2 for receiving a user input command signal indicating a desired dim level in a range between 0 and 100%. If the input command signal indicates a dim level of 100%, the lamp L is operated at 100% of its nominal rating to produce 100% of its nominal light output. If the input command signal indicates a lower dim level, the lamp L is operated with reduced power to produce a reduced light output. The input command signal may for instance be an analog signal or a digital signal, for instance according to the DALI specification, in which case the desired output level as function of the DALI signal is precisely specified. In any case, it is required that the light output is stable, and in the case of a DALI system the light output should meet the DALI specifications.
One solution would be to actually measure the light output, but this is complicated and relatively expensive as it requires the addition of at least one optical sensor. Therefore, in practical systems, the light output is monitored and controlled by monitoring and controlling an electrical parameter, based on the knowledge that the light output is proportional to such electrical parameter.
One electrical parameter that is suitable as control parameter is lamp current; lamp drivers where the lamp current is used as control parameter are known.
Another electrical parameter that is suitable as control parameter is lamp power; lamp drivers where the lamp power is used as control parameter are known.